Help connecting preamp to amp.

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aperayil

Member
Joined 2013
#1
Hello Everyone,



I am just venturing into the audio DIY world, looks like its an amazing ride.

Seems I have hit my first issue already - cant seem to get my preamp and amp to work from the same power source.



Preamp - XH-A901 - NE5532 (with a rectifier at power supply) - InkOcean XH-A901 NE5532 Digital Power Amplifier preamp Tone Board HiFi Fever Grade high and Low bass Adjustment Tuning Board preamp Board : Amazon.in: Electronics

Amplifier - XH-M510 - TDA7498 - Triupy XH-M510 TDA7498 High Power Digital Amplifier Board Car Amplifier : Amazon.in: Electronics

SMPS - LRS-200-24 - Amazon.in: Buy Meanwell Switching Power Supply LRS-200-24 AC-DC 24V 8.8A 90 132VAC 180 264VAC by Switch Mean Well LRS Series Transformer 24VDC Online at Low Prices in India | MEANWELL Reviews & Ratings



Setup - audio source > XH-A901 > XH-M510 > Speakers

The setup works only if the preamp is powered separately, with another SMPS.



Tried bypassing the rectifier on preamp, since it was receiving DC already, but no luck.

Should I have a common ground for both boards or something on those lines?



Pretty sure I might have got something basic wrong in here, hope you guys can point me at the right fix?



Thanks in advance!



Also happen to have a 6J1 tube preamp (AC 12v) Fever 6J1 tube preamp amplifier board Pre-amp Headphone amp 6J1 valve preamp bile buffer diy kits : Amazon.in: Electronics, was planning to put this in to the above setup as an optional preamp - like switch between the 6J1 and NE5532 preamp with a rotary selector.
 
#3
i wrote a long post but it got deleted
while your amp is running on asymmetrical power supply
your preamp seem too, but its possible that on the board it makes symmetrical voltage to power the operational amplifiers, as is typical
so its possible that ground for power amp is not the same as ground for preamp
that could be a problem
 
#5
That.

or you do NOT have the exact preamp you linked to:

From its Q/A section:

Question:
Why is there a diode bridge if the input is 12v dc? or are you supposed to connect it to a 12v ac transformer?
Answer:
This tone board runs on 12-24 v DC, not AC, and there is no diode bridge, you may be refering to some other product?
Click to expand...

You claim:
Tried bypassing the rectifier on ppreamp, since it was receiving DC already, but no luck.

Should I have a common ground for both boards or something on those lines?
Click to expand...
Not sure what you actually did.
 
#6
The diode bridge is clearly visible in the product pictures and the routing can be seen in the underside view.
Presumably reverse supply protection?

Note, the 'answer' was from a customer, not the supplier nor the board manufacturer.

So back to the op. The pre-amp has 24V DC supply that is not referenced to signal ground.
The amp has 24V DC supply that is referenced to signal ground.

The op has the system working with 2 separate supplies.
Can the system be operated with a single supply?
I think not.
I think also the op must ensure the pre-amp supply output is floating and not connected to house ground.
 
#7
Thanks for the feedback russc, adason and JMFahey and sorry for my late response.

Yes ruscc, both pre-amp and amp are rated to work at 24v DC.

I have the same preamp shown in the link, with the diode bridge. Like russc mentioned the diode bridge seems to more of a reverse supply protection.
By bypassing the rectifier (diode bridge) - meant supplying DC voltage after it.

Trying to clarify the setup and some observations made -
Amp - A
Preamp - P
SMPS - S1, S2

A on S1 & P on S2 - works - multimeter shows NO connectivity between preamp's source/output signal ground with supply ground
A on S2 & P on S1 - works - multimeter shows NO connectivity between preamp's source/output signal ground with supply ground
A & P on S1 - doesn't work - multimeter shows connectivity between preamp's source/output signal ground with supply ground
A & P on S2 - doesn't work - multimeter shows connectivity between preamp's source/output signal ground with supply ground

So i guess adason and russc nailed it, the setup works only if pre-amp ground is isolated from the amp & signal ground.

That leads to my next question, apologies if its stupid -
is there a way to decouple/isolate ground between the preamp and amp even if they are on the same power source?
like a cap on the ground signal between the pre-amp and amp and DC isolator on the preamp supply?
 
#8
You need separate supplies.
A pre-amp circuit can be designed to operate with supply negative connected to signal ground.
However, as you are stuck with the board you have, this is not practical.
Think of it as a bonus - the pre-amp supply will be free of any supply fluctuations caused by the power amp.
Having separate supplies also allows you to use a higher voltage for the power amp for more power.
 
#9
Thanks russc, appreciate the explanation, I will go ahead with separate power sources for the setup.
Will build a linear power supply for the pre-amp and use the SMPS for amp at a higher voltage.

Just in case it interests you, made another observation -
The setup works from the same power source, if the ground wire between pre-amp and amp is left open i.e. not connected. Only left/right channel wires are connected between the pre-amp and amp.
But guess that made a ground-loop, it has a hum/buzz which goes up with volume.

I will get started with building a separate power source for the preamp, have a spare 12v 1A transformer.

Thanks again everyone, I will share pictures if I get to complete this setup with the tube pre-amp as a switchable/optional pre-amp.
 
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